1. Field of the Invention and Description of the Related Art
The present invention relates to a circuit for compressing an image signal, and more particularly relates to a circuit for compressing an image signal picked-up by a color television camera such that a maximum value of the image signal does not exceed a predetermined level.
An image pick-up device of the television camera has a very wide dynamic range and sometimes its dynamic range amounts to 400 to 500% of that of an image signal processing system by means of which the image signal generated by the pick-up device is processed. In order to process the image signal having such a wide dynamic range, there is provided a so-called auto-knee circuit which compresses the image signal non linearly in the television camera.
FIG. 1 is a schematic view showing a television camera including a known auto-knee circuit. An imaging lens 61 forms an image of an object 62 on an image pickup device 63. An image signal generated by the pick-up device 63 is processed by a front-stage image processing circuit 64. An image signal processed by the front-stage image processing circuit 64 is then supplied to an autoknee circuit 65 and is compressed thereby. A compressed image signal generated by the auto-knee circuit 65 is furthe processed by a rear-stage image signal processing circuit 66.
There have been proposed auto-knee circuits having various constructions.
For instance, in Japanese Utility Model Publication Kokai Sho No. 60-22076, there is disclosed a known auto-knee circuit, in which a diode is connected in series with a field effect transistor whose conduction is controlled by a peak level of an input image signal and a reverse bias is applied to the diode to set a compression start level. Futher, in Japanese Utility Model Publication No. 63-22765, there is described another known auto-knee circuit for compressing red, green and blue color signals, in which a maximum value of the color signals is detected and gains of amplifiers for amplifying the respective color signals are controlled by the detected maximum value. Then, the input color signals are compared with respective amplified color signals and smaller color signals are selected as compressed color signals. In this manner, portions of the input color signals which exceed a predetermined level can be compressed smoothly.
Moreover, in the television camera illustrated in FIG. 1 the front-stage image signal processing circuit 64 is provided before the auto-knee circuit 65, so that this front-stage image signal processing circuit has to be constructed to be able to process the image signal having the very wide dynamic range. Therefore, the supply source voltage must be made high and a power consumption is liable to be large.
In the known auto-knee circuit disclosed in Japanese Utility Model Publication Kokai Sho No. 60-22076, the compression factor is constant, so that the decrease in the contrast is not recognized. However, a knee point is fixed by a variable resistor, and thus the peak value of the output compressed image signal is undersirably changed.
In the known auto-knee circuit described in Japanese Utility Model Publication No. 63-22765, the peak value of the input image signal is detected and the gain of the amplifier is controlled in accordance with the detected peak value. When the level of the input image signal is excessively large, the compression factor becomes too large so that the contrast of the compressed portion is decreased. This operation is similar to that of a clipping circuit having a constant clipping level. Futher the circuit construction becomes complicated and expensive due to the provision of AGC.